Inkjet printers are one type of apparatus for depositing drops on a substrate. Ink jet printers can include a jetting assembly having one or more printhead modules. Printhead modules include an ink path linking an ink supply with a nozzle path. In some systems, ink is supplied to the jetting assembly from a remote ink supply. The nozzle path terminates in a nozzle opening from which ink droplets are ejected. Ink droplet ejection is typically controlled by pressurizing ink in the ink path with an actuator, which may be, for example, a piezoelectric deflector, a thermal bubble jet generator, or an electrostatically deflected element. Ink in the ink supply that feeds the nozzle path can be held under a negative pressure. This negative pressure can reduce leakage of ink from a nozzle opening when the nozzle is not activated.
A typical printhead module has an array of ink paths with corresponding nozzle openings and associated actuators. Droplet ejection from each nozzle opening can be independently controlled. In a drop-on-demand printhead module, each actuator is fired to selectively eject a drop at a specific pixel location of an image as the jetting assembly and a printing substrate are moved relative to one another. In high performance printhead modules, the nozzle openings typically have a diameter of 50 microns or less, e.g. around 25 microns, are separated at a pitch of 100-300 nozzles/inch, have a resolution of 100 to 3000 dpi or more, and provide drops with a volume of about 1 to 120 picoliters (pl) or less. Drop ejection frequency is typically about 10 kHz or more.
A piezoelectric actuator has a layer of piezoelectric material, which changes geometry, or bends, in response to an applied voltage. The bending of the piezoelectric layer pressurizes ink in a pumping chamber located along the ink path. Piezoelectric ink-jet printhead modules are also described in Fishbeck et al U.S. Pat. No. 4,825,227, Hine U.S. Pat. No. 4,937,598, Moynihan et al. U.S. Pat. No. 5,659,346 and Hoisington U.S. Pat. No. 5,757,391, the entire contents of which are hereby incorporated by reference.